The present invention relates to an analyzer for ions in a liquid, and particularly to an apparatus suitable for analyzing ions in an effluent from a separation column.
When a liquid containing ions is discharged from a capillary to which a high voltage is applied, the liquid is in a conical form, and charged droplets are emitted with a good directivity from the tip of the conical form. A phenomenon wherein ions contained in a liquid are drawn out as droplets therefrom and nebulized by an electric field is known as EHD (Electrohydrodynamic Ionization). This phenomenon requires no heating of the liquid during nebulization and thus causes no thermal decomposition of thermally labile substances such as amino acids and nucleic acids, which are structural materials of protein and genes, respectively, and which are known as polar molecules.
This technique is capable of spraying a liquid at a rate of several .mu.l/min. However, the maximum allowable flow rate of this technique is too low to be applied to an ion extractor used for mass-analyzing ions in an effluent discharged from a separation column used in liquid chromatography. It is therefore necessary to split an effluent from a semi-micro column (&lt;100 .mu.l/min) or a packed column (&gt;100 .mu.l/min) in 100 to 1000 parts. Such a split, however, cannot be easily performed and causes a significant deterioration of the overall sensitivity of the system.
An improvement of the above-mentioned EHD ionization has been proposed. This is described in Analytical Chemistry, Vol.59, No.22, Nov. 15, 1987, pp. 2692-2646.
In this improved method, a fused-silica capillary having an internal diameter of 50 .mu.m is inserted into a stainless steel capillary having an internal diameter of 0.2 mm, these capillaries being further inserted into a Teflon tube having an internal diameter of 0.8 mm. Dry nitrogen gas is caused to flow at a rate of 216 m/sec and at a pressure of 2.5 atm between the stainless steel capillary and the Teflon tube. A voltage of 3 KV is applied to the stainless steel capillary, and a voltage of 600 V is applied to an electrode opposing the stainless steel capillary. It has been reported that a fine mist: like smoke is generated from the fused-silica capillary when a liquid is caused to flow therethrough at several tens of .mu.l/min.
It is thought that, in this technique, the nitrogen gas contributes to the stabilization of a conical form of liquid and the micronization of droplets owing to collision therewith, this being the main cause for an increase in the flow rate.
However, the results of experiments show that it is difficult to stably spray a liquid at a flow rate of 100 .mu.l/min or more and extract ions therefrom even by this technique. This technique is therefore unsatisfactory for a packed column (100 .mu.l/min or more) which is the most frequently used in liquid chromatography.